The present application is based on Japanese Patent Application No. 2007-130965, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical fiber vibration sensor, more particularly, to an optical fiber vibration sensor with excellent detection sensitivity.
2. Related Art
As a conventional sensor for detecting physical external turbulence (vibration, impact) applied to an optical fiber by measuring a variation in an intensity of an interference light generated by an interference of a counter clockwise light and a clockwise light propagated and returned through a looped optical fiber, a Sagnac interference vibration sensor as shown in FIG. 7 has been known.
In FIG. 7, the optical fiber vibration sensor 101 comprises a light source 102, an optical receiver 103, a fiber loop part 104 comprising an optical fiber, an optical dividing/coupling part for inputting signal lights outputted from the light source 102 into the fiber loop part 104 in a counter clockwise direction and a clockwise direction and interfering the signal lights in the counter clockwise direction and the clockwise direction outputted from the fiber loop part 104 to provide an interference light to the optical receiver 103, and a signal processing unit 106 for processing a received signal of the interference light received at the optical receiver 103, in which the optical fiber composing the fiber loop part 104 is installed as an optical fiber for detecting a vibration into a vibration measuring object.
By installing the fiber loop part 104 at a fence, when the fence is vibrated, it is possible to obtain a received light signal similar to that obtained when a rotation angle velocity is generated in the fiber loop part 104. In the Sagnac interference vibration sensor, it is not possible to detect the number of vibration or an absolute value of vibration amplitude. However, it is possible to detect that a specific article hits on the fence, by previously measuring a variation in the received light signal when the specific article hits on the fence to provide a sample for comparison, and comparing the variation in the received light signal that is detected later with the sample for comparison. This system can be applied to detect that a person shook the fence in a restricted zone, a private residence or the like as well as a protective wall for a road or a railway is shaken by a fallen stone. Japanese patent Laid-Open No. 2003-247887 (JP-A-2003-247887) discloses an example of the Sagnac interference vibration sensor.
However, the Sagnac interference vibration sensor disclosed by JP-A-2003-247887 has the sensitivity characteristic that the sensitivity is varied depending on a location in the fiber loop part 104. This phenomenon is caused by that the lights divided in the optical dividing/coupling part 105 and propagated in the counter clockwise direction and the clockwise direction pass at a substantially same time through a part in vicinity of a middle point of the fiber loop part 104 in a loop length direction. In particular, the sensitivity is 0 at the middle point of the fiber loop part. Accordingly, the vibration detecting sensitivity is not uniform depending to the location in the vibration measuring object in which the fiber loop part 104 is installed.